1. Field of the Invention
The present invention relates to the field of power electronics. In particular, it relates to a controllable high-power electron tube in the form of a tetrode having an output power P.sub.0 of at least 100 kW, which high-power electron tube comprises a cathode, a control grid, a screen grid and an anode in coaxial cylindrical arrangement.
Such a high-power electron tube is known, for example, by the type designation CQK 50-2 from the printed document Brown Boveri Review 66, 1979 (1), pages 40-42. The typical application of this tube in a broadcast transmitter is described in the printed document Brown Boveri Review 67, 1980 (3), pages 215-219.
2. Discussion of Background
High-power electron tubes of the type initially mentioned are usually employed as output stage tubes in broadcast transmitters with amplitude modulation (AM), particularly in the short-wave band (about 3.9-26.1 MHz). Such a broadcast transmitter comprises an AF and an RF section.
The AF section provides for the processing and power amplification of the AF signal to be transmitted, which is then applied to the anode of the output stage tube in the case of the usual anode modulation. In the RF section, the carrier-frequency oscillator with the subsequent driver stage provides a power-amplified carrier signal which passes to the control grid of the output stage tube and, together with the anode voltage, which oscillates at the rate of the AF signal, emits the desired AM signal to a load, the antenna.
Since such broadcast transmitters usually operate within a power range of more than 50 kW up to a few 100 kW output power, the efficiency, that is to say the ratio between power used and usable power, plays a central role in the development and design of such a transmitter. The output stage tube takes a significant share of the total efficiency of the transmitter which can be greater than 70%.
Its efficiency, the so-called anode efficiency, is proportional to, among other things, the expression 1-(u.sub.s /u.sub.a0, where u.sub.s (also called U.sub.ar) is the residual voltage which cannot be modulated and u.sub.ao (also called U.sub.a) is the anode direct voltage. With the residual voltage remaining constant, the anode efficiency therefore rises with increasing anode direct voltage (Brown Boveri Review 71, 1984(5), page 199).
In all cases, a good anode efficiency therefore requires a high anode direct voltage u.sub.ao so that the unmodulatable residual u.sub.s remains relatively small by comparison (see also: Meinke/Gundlach, Taschenbuch der Hochfrequenztechnik, (Pocket Book of Radiofrequency Engineering), 3rd edition, Springer-Verlag 1968, pages 1035-1037). The usual operating voltages in large transmitters with high-power tetrodes in the RF output stage are therefore between 10 and 14 kV (see also: Meinke/Gundlach, Taschenbuch der Hochfrequenztechnik (Pocket Book of Radiofrequency Engineering) 4th edition, Springer-Verlag 1986, page P9).
The following two shortwave transmitters can be used as examples for these values achieved in the prior art:
(1) The 250-kW shortwave transmitter described in the printed document Brown Boveri Review 69, 1982 (6), pages 212-217, the RF output stage of which is equipped with a high-power tetrode of the BBC CQK 350-1 type. This tetrode operates in class C mode with anode modulation, with an anode direct voltage of 14 kV, a screen grid direct voltage of 1300V and a control grid direct voltage of -900V and has an efficiency of 85.2%.
(2) The 100-kW shortwave transmitter described in the printed document Brown Boveri Review 67, 1980 (3), pages 215-219, described initially, the RF output stage of which is equipped with a high-power tetrode of the BBC CQK 50-2 type. This tetrode operates in class C mode with anode modulation, with an anode direct voltage of 11 kV, a screen grid direct voltage of 800V and a control grid direct voltage of -600V (see also BBC short data catalog electron tubes, printed document No. CH-E 3.30475.8 D/F/E/S of 1982/83).
The comparatively high anode voltages, in connection with the anode modulation, require correspondingly designed modulation amplifiers which have to supply output voltages from 0 to 28 kV with an anode direct voltage of 14 kV.
If, for example, a pulse step modulator PSM, that is to say a digital switched-mode amplifier, is used as modulation amplifier, 32 switching stages, for example, are needed within this PSM, the output voltages of which add up to the desired anode voltage (Brown Boveri Tech. 74, 1987(6) , pages 296-302). Since each individual one of these 32 high-power switching stages requires the corresponding space, separate cabinets must be provided for the PSM in the transmitter.
But the high anode direct voltage and the voltage strength required for it also leads to increased space requirement for other components of the transmitter circuit, with the result that the transmitter overall is inevitably very costly to produce.